A number of embodiments of fluid control valves exist, forming a large body of valve art. The particular device that is the subject of this invention is a further development of rotary control valves.
A ball valve is a rotary control valve, comprised generally of a body, a rotating plug, seats, a bonnet, bearings, and seals. The assembly is placed in a conduit and used to control the flow of fluid, liquid or gas, through the flow bore. The components operate together such that the ball (a form of plug), usually rotating through 90.degree., can move from a full open (maximum flow through valve) position to a full closed (minimum or no flow through the valve) position. The rotating ball can also be positioned between either the full open or the full closed position, i.e., in one or more intermediate positions. The positioning of the ball or plug in a valve can be by means of a manual device, such as a lever or worm gear, operating on an extension of the ball or plug outside of the pressure boundary of the valve body/bonnet. The positioning can also be accomplished by non-manual means, such as by a pneumatic, hydraulic, or electrically powered actuator. The actuator positions the plug in response to an electrical or fluid signal. A valve plus a control device can be used to proportionally control the flow of fluid in response to an input signal.
As fluid passes through a rotary control valve assembly, and specifically past the upstream seat and the downstream seat, if the ball or plug is in a partially opened position the fluid undergoes a simultaneous pressure drop and velocity increase. This phenomena occurs if the ball/plug is in any intermediate position between the full open and full closed positions sufficient to permit fluid flow. A pressure and velocity profile through a partially opened, typical rotary control ball valve is exhibited in FIG. 1. These velocity and pressure profiles have been described in fluid flow texts as well as in the published data of a number of valve companies. Essentially, the fluid must obey the following fluid flow law: ##EQU1##
The fact that the pressure and velocity profiles take marked dips or jumps when traversing the seats of the valve, as illustrated in FIG. 1, can be disadvantageous. For example, any dip in pressure below the vapor pressure of a particular liquid fluid can cause the formation of gas bubbles in the fluid. As the fluid pressure then increases (downstream of the locations where the pressure level is the lowest and the velocities are the highest) the vapor bubbles then collapse. The phenomena is known as cavitation. If gas flow only is present, the locations within the valve embodiment that cause large velocity steps (increases) generate noise and vibration. Both cavitation and vibration are detrimental to the valve and piping components, sometimes doing extensive damage. Noise can be detrimental to the environment. In fact, the maximum noise to be tolerated, measured in decibels, can form a design limitation for an application of a valve.
Noise and/or vibration abating valves are ones that allow or cause the aforementioned steps in pressure drop, or jumps in velocity increase, to be accomplished in a multiplicity of small steps rather than in one or two large steps. The fact that pressure increases after passing each restriction point, and that the amount of this pressure increase is relatively large (high) for a typical rotary control valve generating the data of FIG. 1, has given the term "high recovery" to this rotary valve. As mentioned above, the "high pressure recovery" phenomena may cause cavitation or noise generation. If by mechanically modifying the valve flow passages a more idealized flow through the valve can be achieved, then the adverse affects of the "high recovery" basic valve can be modified to qualify the valve as a "low recovery" device.
Other types of valves can be "low recovery" devices. Such devices, for example, as "chokes," commonly used in oil and gas production, are considered to be inherently "low recovery" devices.
The rotary control valve, notwithstanding its typical high recovery characteristics, does have major advantages compared to other types of control valves. One of the major advantages is that it can successfully control fluid over a wide range of flows. Another way of stating this is that the ratio of maximum to minimum flow (called "rangeability") of a rotary control valve can be very high. The "rangeability" can be a ratio of eighty to one (80/1) up to three hundred to one (300/1) in a ball valve.
An ideal embodiment of a rotary control valve combines the advantage of "wide rangeability" with the desirability of "low recovery" so that the device can be utilized over a wide range of flows without the adverse affects of cavitation or noise.
Several fluid flow control embodiments have combined rotary control valves with trim modifications to reduce the pressure recovery characteristics. The approaches used have either involved an upstream plate (to cause a pressure drop before the rotary element), or a trim within the rotating ball (or plug) to create a higher impedance to flow (multiple pressure drops) when the ball is in the throttling (intermediate) position, or a downstream trim to cause a pressure drop at the valve outlet. Some embodiments have used two of these in combination.
U.S. Pat. No. 3,630,229 (Nagle) recognizes that a multiple arrangement of parallel tubes combined with a rotating ball element is a more effective means of reducing noise and cavitation than a full-sized outlet bore would be. Similar concepts have been used by other manufacturers (Fisher and Neles, for example) in the form of tube bundles on the outlet side of segmented ball valves or inlet baffles combined with a special trim within the ball. A number of manufacturers have recognized that the noise/cavitation of a valve can be reduced if not all of the pressure drop is taken across the moving control element. Inlet and outlet trims, consisting of perforated tubes, tube bundles, orifices, and even outlet diffusors have been used in, and/or in configuration with, valve control elements in order to divide the pressure drops across the valve into more steps. In gas flows, it is not uncommon to utilize expanded outlets for the valve to increase the outlet area, thus reducing the velocity (and noise) that would otherwise result.
This invention teaches a novel arrangement of inlet/outlet trims and a matching rotating plug/trim that includes the alignment of the openings of cylindrical ducts located in the inside face of a fixed inlet trim, (and/or of a fixed inlet and outlet trim) with the openings of cylindrical (intercommunicating) ducts in the face of a rotary control plug. The ducts and the openings in the faces of the fixed trim and rotating elements combine to greatly increase the effectiveness of the valve in reducing noise and cavitation. The specific reason for the improved performance is the added effectiveness of the "series" and "paralleling" combination chokes operating in conjunction with the variable impedance of a rotating plug/trim. Such an arrangement could include, in addition, enlarging the opening capacity of the ducts in the downstream face of the outlet trim.
It has been found that the combination in the present invention of cylindrical ducts in inlet/outlet trims and matching intercommunicating cylindrical ducts in a rotating plug/trim reduces noise generation substantially. This is achieved by combining the behavior of "chokes" and the "variable impedance" of a rotating plug/trim. The assembly acts as a combination of variable chokes. In particular, in the near open position, when the rotating plug/trim becomes markedly less effective in reducing noise and cavitation, the near alignment of the cylindrical ducts allows each to act effectively as a variable choke in the flow range where the rotating plug/trim, by itself, is least effective.
The invention further permits particles, up to the size of the duct diameter, to pass through the valve, thereby permitting the valve to be used in applications that involve contaminated fluids. It has been found that the duct diameter may be 1/4 of an inch or larger will not significantly affect the low recovery characteristics of the valve. In certain fluids, particularly liquids, the arrangement can be bi-directional or multi-directional.
Further advantages of the design as specified in the preferred embodiment will be readily appreciated. For instance, the design provides for the preservation of the alignment of the inlet and outlet trims to assure the line up of the duct openings. The design provides for the removal and reinstallation of each of the trim components through the bonnet of the valve. The design provides for expansion and contraction of the trim elements relative to other portions of the valve assembly. The design arranges that the flow does not bypass the intended trim element ducts. The design allows for using appropriate wear resistant materials for the insertable trims.